This invention generally relates to voltage measurement devices, and more particularly to digital voltmeters capable of measuring AC and DC voltages with automatic selection of the proper measurement function so as to provide a safe measuring instrument. The digital voltmeter of the invention may also provide a measuring instrument which is substantially insensitive to the effects of distributed capacitance, for accurate and reliable voltage measurement.
Many situations, from checking and testing of electrical service installations to checking the integrity of residential wiring, require the use of voltage measuring equipment capable of measuring the magnitude of an AC line voltage, DC voltage or combination thereof. It should be evident that in the verification of electrical service voltage, very high voltages may be encountered, making routine power measurement a possibly dangerous activity. For providing such measurement capabilities, there have been developed a number of AC/DC voltage reading voltmeters or multi-function meters adapted to measure voltage in conjunction with other electrical parameters.
In this respect, various AC and DC analog voltmeters have been developed which operate with a galvanometer type meters to indicate the magnitude of the voltage being measured. Many of these analog type instruments are deficient in that they have low input impedance and therefore require a substantial amount of electrical current to deflect the meter, which will distort the voltage measurement in many cases. Digital voltmeters have also been developed which may avoid the inaccuracy of some analog meters, but in turn have other recognized deficiencies.
Both analog and digital voltmeters are subject to inaccuracy, are bulky, fragile and the use thereof is possibly dangerous. Many such meters include range and function selection switches which are manipulated by the user. The voltmeter must be capable of measuring voltages over a wide range of magnitudes, which will require the user to have knowledge of what voltages are to be expected. If the wrong function or range is selected on a voltmeter, there is the possibility that an over-voltage could cause the measuring instrument to explode upon application to a voltage source. Additional problems can be encountered when an electrician or other user attempts to use an AC voltmeter to clear a DC line under a misconception of whether a particular line carries AC or DC line voltage, or a DC voltage with an AC ripple.
In other situations, such as checking the integrity of residential wiring, the measurement of the voltage between the line and load connections of a meter-base with the residential switch in the open position is performed. In these and other situations, distributed capacitance present on the line may inhibit accurate measurements with a voltmeter. Most digital voltmeters as well as a few analog voltmeters cannot be used where significant capacitive effects are present. A "parasitic" current flows through the meter which returns to ground via the distributed free-space and interconductor capacitance of the load wiring. Although sensitive voltmeters are able to measure this "parasitic" current, the presence of this current will result in ambiguous readings as to the condition of the load wiring. A low impedance voltmeter may avoid the effects of distributed capacitance to some degree, but have the deficiencies as mentioned previously with respect to accuracy of voltage measurements.
In yet other situations, electrical continuity measurements are desired to be made with a voltmeter, but such measurements may compromise the voltage measurement capabilities of the meter. For example, continuity measurements may be made with a solenoid type meter, but accuracy of the voltage measurements is compromised. The prior art voltage measuring equipment has generally been found to not be as reliable or as safe as desired, and has lacked the flexibility to perform variable measurement operations in an easy and accurate manner.